The two main groups of research methods are observation studies and intervention studies. In this review two examples of observation studies are discussed: cohort studies and case - control studies, as well as an example of intervention studies: randomized controlled trials.In cohort studies, the researchers recruit subjects without the disease under investigation, who are exposed to risk factors for this specific disease. The subjects are followed-up prospectively and the diseases that emerge are recorded systematically. The results are analyzed at the end of the study. The main difference between cohort studies and case - control studies is that in the later the disease under investigation has already been diagnosed. The researchers recruit a patient and a control group, indicate the incidence of risk factors in the two groups and compare the ratios. In randomized controlled trials, the subjects are randomized into intervention and control groups. Ideally, neither researchers nor patients must know if the patients have randomized into the intervention or the control group. Finally, a comparison between the two groups takes place in order to search for statistically significant differences.The three types of studies mentioned in this review are commonly used in medical research. Medical personnel who are interested in research should learn these methods thoroughly. Those doctors who are not involved in medical research should familiarize themselves with the inherent strengths and weaknesses of different research study designs so that they can be more critical when appraising published articles.